Suk-Jin Yoon

Galaxy evolution explorer ultraviolet color-magnitude relations and evidence of recent star formation in early-type galaxies

We have used the Galaxy Evolution Explorer UV photometric data to construct a first near-UV (NUV) color-magnitude relation (CMR) for the galaxies preclassified as early-type by Sloan Digital Sky Survey studies. The NUV CMR is a powerful tool for tracking the recent star formation history in early-type galaxies, owing to its high sensitivity to the presence of young stellar populations. Our NUV CMR for UV-weak galaxies shows a well-defined slope and thus will be useful for interpreting the rest-frame NUV data of distant galaxies and studying their star formation history. Compared to optical CMRs, the NUV CMR shows a substantially larger scatter, which we interpret as evidence of recent star formation activities. Roughly 15% of the recent epoch (z < 0.13) bright [M(r) < -22] early-type galaxies show a sign of recent (1 Gyr) star formation at the 1%-2% level (lower limit) in mass compared to the total stellar mass. This implies that low-level residual star formation was common during the last few billion years even in bright early-type galaxies.

The effect of environment on the ultraviolet color-magnitude relation of early-type galaxies

We use GALEX near-UV (NUV) photometry of a sample of early-type galaxies selected in the SDSS (Sloan Digital Sky Survey) to study the UV color-magnitude relation (CMR). NUV − r color is an excellent tracer of even small amounts (~1% mass fraction) of recent (≲1 Gyr) star formation, and so the NUV − r CMR allows us to study the effect of environment on the recent star formation history. We analyze a volume-limited sample of 839 visually inspected early-type galaxies in the redshift range 0.05 < z < 0.10 brighter than M_r of –21.5 with any possible emission-line or radio-selected active galactic nuclei (AGNs) removed to avoid contamination. We find that contamination by AGN candidates and late-type interlopers highly bias any study of recent star formation in early-type galaxies and that, after removing those, our lower limit to the fraction of massive early-type galaxies showing signs of recent star formation is roughly 30% ± 3% . This suggests that residual star formation is common even among the present day early-type galaxy population. We find that the fraction of UV-bright early-type galaxies is 25% higher in low-density environments. However, the density effect is clear only in the lowest density bin. The blue galaxy fraction for the subsample of the brightest early-type galaxies, however, shows a very strong density dependence, in the sense that the blue galaxy fraction is lower in a higher density region.

Super-helium-rich populations and the origin of extreme horizontal-branch stars in globular clusters

Recent observations for the color-magnitude diagrams (CMDs) of the massive globular cluster ω Centauri have shown that it has a striking double main sequence (MS), with a minority population of bluer and fainter MS stars well separated from a majority population of MS stars. Here we confirm, with the most up-to-date Y2 isochrones, that this special feature can only be reproduced by assuming a large variation (ΔY = 0.15) of primordial helium abundance among several distinct populations in this cluster. We further show that the same helium enhancement required for this special feature on the MS can by itself reproduce the extreme horizontal-branch (HB) stars observed in ω Cen, which are hotter than normal HB stars. Similarly, the complex features on the HBs of other globular clusters, such as NGC 2808, are explained by large internal variations of helium abundance. Supporting evidence for the helium-rich population is also provided by the far-UV (FUV) observations of extreme HB stars in these clusters, where the enhancement of helium can naturally explain the observed fainter FUV luminosity for these stars. The presence of super-helium-rich populations in some globular clusters suggests that a third parameter, other than metallicity and age, also influences the CMD morphology of these clusters.

Explaining the Color Distributions of Globular Cluster Systems in Elliptical Galaxies

The colors of globular clusters in most large elliptical galaxies are bimodal. This is generally taken as evidence for the presence of two cluster subpopulations that have different geneses. However, here we find that, because of the nonlinear nature of the metallicity-to-color transformation, a coeval group of old clusters with a unimodal metallicity spread can exhibit color bimodality. The models of cluster colors indicate that horizontal-branch stars are the main drivers behind the empirical nonlinearity. We show that the scenario gives simple and cohesive explanations for all the key observations and could simplify theories of elliptical galaxy formation.

Better age estimation using ultraviolet–optical colours: breaking the age–metallicity degeneracy

We demonstrate that the combination of GALEX ultraviolet (UV) photometry in the far-UV (FUV; ~1530 A) and near-UV (NUV; ~2310 A) passbands with optical photometry in the standard U, B, V, R, I filters can efficiently break the age–metallicity degeneracy. We estimate well-constrained ages, metallicities and their associated errors for 42 globular clusters (GCs) in M31, and show that the full set of FUV, NUV, U, B, V, R, I photometry produces age estimates that are ~90 per cent more constrained and metallicity estimates that are ~60 per cent more constrained than those produced by using optical filters alone. The quality of the age constraints is comparable or marginally better than those achieved using a large number of spectroscopic indices.

The Hβ Index as an Age Indicator of Old Stellar Systems: The Effects of Horizontal-Branch Stars

The strength of the H? index is computed for the integrated spectra of model globular clusters from the evolutionary population synthesis. For the first time, these models take into account the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity. Our models show that the H? index is significantly affected by the presence of blue HB stars. Because of the contribution from blue HB stars, the H? does not monotonically decrease as metallicity increases at a given age. Instead, it reaches a maximum strength when the distribution of HB stars is centered around 9500 K, the temperature at which the H? index becomes strongest. Our models indicate that the strength of the H? index increases as much as 0.75 ? because of the presence of blue HB stars. The comparison of the recent Keck observations of the globular cluster system in the Milky Way with those in the giant elliptical galaxies NGC 1399 and M87 shows a systematic shift in the H?-metallicity plane. Our models suggest that this systematic difference is explained if the mean age of globular cluster systems in giant elliptical galaxies is several billion years older than the Galactic counterpart. Further observations of globular cluster systems in the external galaxies from the large ground-based telescopes and space UV facilities will enable us to clarify whether this difference is indeed due to the age difference or whether other explanations are also possible.

CCD Photometry of the Classic Second-Parameter Globular Clusters M3 and M13

We present high-precision V, B-V color-magnitude diagrams (CMDs) for the classic second-parameter globular clusters M3 and M13 from wide-field, deep CCD photometry. The data for the two clusters were obtained during the same photometric nights with the same instrument, allowing us to determine accurate relative ages. Based on a differential comparison of the CMDs using the Δ(B-V) method, an age difference of 1.7 ± 0.7 Gyr is obtained between these two clusters. We compare this result with our updated horizontal-branch (HB) population models, which confirm that the observed age difference can produce the difference in HB morphology between the clusters. This provides further evidence that age is the dominant second parameter that influences HB morphology.

Nonlinear Color-Metallicity Relations of Globular Clusters. III. On the Discrepancy in Metallicity between Globular Cluster Systems and Their Parent Elliptical Galaxies

One of the conundrums in extragalactic astronomy is the discrepancy in observed metallicity distribution functions (MDFs) between the two prime stellar components of early-type galaxies—globular clusters (GCs) and halo field stars. This is generally taken as evidence of highly decoupled evolutionary histories between GC systems and their parent galaxies. Here we show, however, that new developments in linking the observed GC colors to their intrinsic metallicities suggest nonlinear color-to-metallicity conversions, which translate observed color distributions into strongly peaked, unimodal MDFs with broad metal-poor tails. Remarkably, the inferred GC MDFs are similar to the MDFs of resolved field stars in nearby elliptical galaxies and those produced by chemical evolution models of galaxies. The GC MDF shape, characterized by a sharp peak with a metal-poor tail, indicates a virtually continuous chemical enrichment with a relatively short timescale. The characteristic shape emerges across three orders of magnitude in the host galaxy mass, suggesting a universal process of chemical enrichment among various GC systems. Given that GCs are bluer than field stars within the same galaxy, it is plausible that the chemical enrichment processes of GCs ceased somewhat earlier than that of the field stellar population, and if so, GCs preferentially trace the major, vigorous mode of star formation events in galactic formation. We further suggest a possible systematic age difference among GC systems, in that the GC systems in more luminous galaxies are older. This is consistent with the downsizing paradigm whereby stars of brighter galaxies, on average, formed earlier than those of dimmer galaxies; this additionally supports the similar nature shared by GCs and field stars. Although the sample used in this study (the Hubble Space Telescope Advanced Camera for Surveys/Wide Field Channel, WFPC2, and WFC3 photometry for the GC systems in the Virgo galaxy cluster) confines our discussion to R Re for giant ellipticals and 10 Re for normal ellipticals, our findings suggest that GC systems and their parent galaxies have shared a more common origin than previously thought, and hence greatly simplify theories of galaxy formation.

On the Origin of Bimodal Horizontal Branches in Massive Globular Clusters: The Case of NGC 6388 and NGC 6441

Despite the efforts of the past decade, the origin of the bimodal horizontal branch (HB) found in some globular clusters (GCs) remains a conundrum. Inspired by the discovery of multiple stellar populations in the most massive Galactic GC, ω Centauri, we investigate the possibility that two distinct populations may coexist and are responsible for the bimodal HBs in the third and fifth brightest GCs, NGC 6388 and NGC 6441. Using the population synthesis technique, we examine two different chemical self-enrichment hypotheses in which a primordial GC was sufficiently massive to contain two or more distinct populations as suggested by the populations found in ω Cen: (1) the age-metallicity relation scenario in which two populations with different metallicity and age coexist, following an internal age-metallicity relation, and (2) the super-helium-rich scenario in which GCs contain a certain fraction of helium-enhanced stars, for instance, the second-generation stars formed from the helium-enriched ejecta of the first. The comparative study indicates that the detailed color-magnitude diagram morphologies and the properties of the RR Lyrae variables in NGC 6388 and NGC 6441 support the latter scenario: i.e., the model which assumes a minor fraction (~15%) of helium excess ( -->Y 0.3) stars. The results suggest that helium content is the main driver behind the HB bimodality found most often in massive GCs. If confirmed, the GC-to-GC variation of helium abundance should be considered a local effect, further supporting the argument that age is the global second parameter of HB morphology.

GALEX Ultraviolet Photometry of Globular Clusters in M31: Three-Year Results and a Catalog

We present ultraviolet (UV) photometry of M31 globular clusters (GCs) found in 23 Galaxy Evolution Explorer (GALEX) images covering the entirety of M31. We detect 485 and 273 GCs (and GC candidates) in the near-ultraviolet (NUV) and far-ultraviolet (FUV), respectively. The UV properties of GCs have been analyzed using various combinations of UV–optical and optical–optical colors. Comparing M31 data with those of Galactic GCs in the UV with the aid of population models, we find that the age ranges of old GCs in M31 and the Galactic halo are similar. Three metal-rich ([Fe/H] > − 1) GCs in M31 produce significant FUV flux making their FUV–V colors unusually blue for their metallicities. These are thought to be analogs of the two peculiar Galactic GCs, NGC 6388 and NGC 6441, with extended blue HB stars. Based on the models incorporating helium enriched subpopulations in addition to the majority of the population that have a normal helium abundance, we suggest that even small fraction of super-helium-rich subpopulations in GCs can reproduce the observed UV bright metal-rich GCs. Young clusters in M31 show distinct UV and optical properties from GCs in Milky Way. Population models indicate that their typical age is less than ~2 Gyr and is consistent with the age derived from the most recent high-quality spectroscopic observations. A large fraction of young GCs have the kinematics of the thin, rapidly rotating disk component. Most GCs with bulge kinematics show old ages. The existence of young GCs on the outskirts of M31 disk suggests the occurrence of a significant recent star formation in the thin-disk of M31. We detect 12 (10) intermediate-age GC candidates in NUV (FUV) identified by previous spectroscopic observations. On the basis of comparing our UV photometry to population models, we suggest that some of spectroscopically identified intermediate-age GCs may not be truly intermediate in age, but rather older GCs that possess developed HB stars which contribute to enhanced UV flux as well as Balmer lines.